Of the many optical methods used to investigate semiconductors and semiconductor microstructures, one of the most useful is electromodulation (EM). In electromodulation, the periodic modulation of an applied electric field produces sharp features in the reflectivity spectrum of the material at photon energies corresponding to interband (intersubband) transitions. These derivative-like resonances can be used to study and characterize many of the important properties of semiconductors (bulk thin film), semiconductor surfaces/interfaces, semiconductor microstructures (single quantum wells, multiple quantum wells, superlattices and heterojunctions) as well as actual device structures.
Electromodulation can be accomplished in several ways, including contact and contactless modes. Four common contact configurations, designated as electroreflectance (ER) can be divided into "longitudinal" and "transverse" categories. The "longitudinal" method can be applied in the semiconductor-electrolyte, metal-insulator-semiconductor, Schottky barrier or PIN configurations. In the latter the sample of interest is placed in the insulating region of a PIN diode. The former is the most widely used form of electroreflectance because of ease of implementation as the sample surface requires no special preparation. However, it can be used only over a limited temperature range (300K to 150K) and often offers less control over the space charge field owing to chemical passivation or dissolution effects. Electrolyte electroreflectance (EER) can be employed for depth profiling measurements with the proper choice of electrolyte and electrochemical conditions. Schottky barrier, metal-insulator-semiconductor and PIN methods can be used at low temperatures to reduce lifetime broadening. The PIN configuration produces a constant electric field as opposed to the position-dependent field of the other "longitudinal" modes. However, the sample must be specially fabricated in order to employ this mode.
In the "transverse" mode, two metal electrodes are evaporated on the surface of the sample and electromodulation is produced by applying a modulated high voltage (.about.1 kV) across the gap (.about.1 mm.). However, this technique can only be used on materials with resistivities greater than about 10.sup.8 ohm-cm.
Contactless electromodulation can be performed using photoreflectance (PR) or electron-beam electroreflectance (EBER). The method of photoreflectance is not only contactless but requires no special mounting of the sample. It can be used in any transparent medium under a variety of conditions. Modulation of the electric field in the sample is caused by photo-excited electron-hole pairs created by a pump source, such as a laser or other light source, which is chopped at a frequency .OMEGA..sub.m. These photo-injected electron-hole pairs modulate the built-in electric field of the semiconductor or semiconductor microstructure. The photon energy of the pump source must be above the bandgap of the semiconductor being investigated. A typical pump is a 5 mW He-Ne laser, except at high temperatures where a more powerful beam must be used. In electron-beam electroreflectance, the pump beam is replaced by a modulated low energy electron beam (.about.200 eV) chopped at about 1 kHz. However, the sample and electron gun must be placed in an ultra-high vacuum chamber.
Recently, a new version of differential reflectivity has been reported by M. Gal and C. Shwe, Appl. Phys. Lett. 56, 545 (1990) which sometimes contains an electromodulation component. This contactless optical method measures the difference between the reflectivities of two materials (sample/reference). In this approach the sample/reference is mounted so that it performs small oscillations in the plane perpendicular to the incoming probe light beam. If there is a difference in electric field between the sample and reference an electromodulation-like signal can be produced.